Electricity |The electric is the rate of flow of current in a conductor

 

CHAPTER 10 -ELECTRICITY

#In this chapter we will learn about electricity that how elctricity transfer and how heating effect produce ,SI units of Electric current,electric potential ,power ,what is resistance and on which it depends.

So this is only a overview ,now we will understand through definition.

ELECTRIC POTENTIAL:The electric potential at a point is an electric field is define as work done in moving a unit charge from infinity to that point it is denoted by V and unit is Volt.

V=Work done/Charge

V=J/C

Potential differnce-The potential differnce define as amount of work done in moving a unit charge from one poin t to another point.

Potential Difference=Work done/Quantity of charge Move

SI unit Of P.D is Volt

Potential Differnce is measured by an instrument called Voltmeter.

Voltmeter is always connected in parallel accross the two point where the P.D is to be measured.

Q.A voltmeter has resistance ? Why?

=>Because it take the neglegible current.

ELECTRIC CURRENT:The electric is the rate of flow of current in a conductor if a charge of Q coloumb flows through a conductor in  a time T second and magnitude of current (I)

I=Q/t

SI unit of current is Ampere.DEnoted by (A).

Q.How to define 1 Ampere?

>When one coloumb lof charge flow through any cross section of a conductor in one second the electric flowing through it is said to one Ampere  .

Therfore one ampere is equal one coloumb by one second.

NOTE:One mili Ampere is equal to one by thousand Ampere is equal 10^-3.

Current is measured by an instrument called Ameter.

Q.An Ameter is always coonect in series?

Ameter should have very low resistance so that it may not chrage the value of current flowing in the circuit.

Q.A current of 0.5 A is drawn by filament of bulb 10 minutes find the aamount odf electric chrage flows through the circuit?

Ans.t=10 *60sec=600sec

I=Q/t

Q=I*t=0.5*600

Q=300C

Q,How much work done in moving a charge 2C accross two point having potential difference 12V?

Ans.Potenial Differnce=Workdone/Charge

P=W/Q

W=P*Q=12*2

W=24J

Ohm's Law 

In 1827,German Scientist George simon Ohm given this law.

Ohm's law  gives relation between current and potential differnce.According to Ohm's law "At constant temperature ,the cuurent flowing through the conductor is directly proportional t the potential difference accross its end.If 'I' is the current and V is the potential differnce across its end.

V is derctly proportional to Current

V=RI  where R is constant

R=V/I

From this equation Rsistance is the ratio of potential differnce between ends of cnductor and the current flowing through it is a constant quantity called Resistance.

if the potential differnce accross end of conductor is doubled then the current flowing through it is also get doubled.

A Resistance of a conductor

1)The property of a conductor due to which it opposes the flow of a current through it is called resistance.

R=V/I

SI unit of Rsistance is ohm.

GRAPH BETWEEN V AND I

If graph  is a drawn  between voltage and current the graph is formred to be a straight line passing through the origin.

Resistor:Those substance which having  a capabilty it comperatelyhigh resistance called resistor.eg.alloy of nichrom,magnesium etc.

FACTORS ON WHICH RESISTANCE OF CONDUCTOR IS DEPENDS

1)length of s conductor

2)cross section area of conductor

3)Temperature of conductor

4)Nature of the material of a conductor

1)Effect of length of a conductor:it is found by experiment that on increasing length of wire ,its resistance is increase and vice -versa.

2)Effect of cross=-section :it is found by experiment that on increasing cross=-section area  of wire ,its resistance is decrease and vice -versa.

3)Effect of temprtaure:Resistance of all pure metal incerasing thye temperature  but resistance of alloys on effected by the temperature 

4)Effect of nature of a material:Some material having low resistance where as other have high high resistance.

RESISTIVITY:(Specific Resistance)

it is found by an experiment that 

R `is directly proportional to l 

R∝L               --------------------------------------1

      

the resistance of a given conductor is inversly proportional to cross section area 

R is inversly proportional to A    

R $\propto$ $\frac{1}{A}$

 

by equation 1 and 2

R = $p\frac{l}{A}$ (3)

RA = $\rho$l

Or its alternate can be, $\rho$ = $\frac{RA}{l}$

where 

$$
\rho is a constant known as resistivity of conductor of metal it is also known as specific resistivity.

SI unit of resitivity ohm meter.

Resistivity of a conductor do not depend upon its length  and thickness .

It only depends on substance and temperature 

Q.We use copper and aluminium called transmission of electricity .why?

Because it has low resistivity .

Resistivity of an alloy is much more than the pure metal.Heating element of electrical heating appliances such as iron ,toaster,etc are made up of alloys rather thsn pure metal.

NOTE:Tungsten is used for filament of a bulb.

Silver is the best conductor because its resistivity is very low 1.6*10^-8.

The combine resistance of any resistance is connected in series is equal to the sum of individual resistance

R=R1+R2+R3+……

 

Value of current in ammeter is the same through out the circuit.

 

Potential difference at each resistance differs or the sum of the potential difference V=V1+V2+V3+………

 

Applying voltage V= V1+V2  ----------------1

 

According to ohm’s law

V=IR        

V1=I *R1

V2=I* R2

 

By putting the value of v1 and V2 in equation 1

V= I *R1+ I *R2

IR=I(R1+R2)

R=R1+R2

 

v     RESISTORS IN PARALLEL

 

 

A resistances are resistor in parallel .the reciprocal of combine resistance of a number of resistances connected in parallel is equal to the sum of a reciprocal of individual resistance.

 

R is the combine resistance r1 ,R2,R3 are individual resistance

1/R=1/R1+1/R2+1/R3

 

·       Potential difference across the each resistance will be same.

·       Different amount of current  will flow through each resistance depend uopon the value of resistance.

 

 

According to ohm’s law

I=V/R        

I1=V/ R1

I2=V/ R2

 

I=I1+I2

 

V/R=V/R1+V/R2

 

1/R=1/R1+1/R2

 

 

Ø    DISADVANTAGE  of Series Circuit

1)In series circuit if one electrical appliance stop working due to some defect ,when all other appliances  stop working

2)In series circuit all the appliance s have only one  switch due to whichg other appliances cannot be switched off.

3) In series circuit appliances do not get the same voltage .

4)The overall resistance of circuit is increases.

 

Ø    ADVANTAGE  of Parallel Circuit

 

 

1)In parallel circuit if one electrical appliances stop working due to some defect then all other appliances keep working.

2)In Parallel circuit each appliances have  own  switch due to whichg other appliances can be switched .

3) In parallel circuit appliances  get the same voltage .

4)The overall resistance of circuit is decreases.

       HEATING EFFECT OF ELECTRIC CURENT

Ø Source energy continuning gets disperated in the form of heat this is also known as heating effect of electric current,this effect is utilized in devices such as electric heater ,electric iron,etc.

Ø Derivation of formula for the heat produced

 

1)Consider a current flowing through a resistor of a resistance  R.

Let the potential difference across it ends be V,let small ‘t’ be the time during which ‘Q’ flows across .the work done in moving the charge ,we know

V=W/Q

W=V*Q

We know

I=Q/t

Therefore ,

Q=I*t

It implies that,

W=I*R*I*t

W=I^2*R*t

Therefore,

H=I^2*R*t

H is heat produced

Assuming that al the electric bulb are electrical energy consume is converted into the heat energy.

 

We can write heat produced in placed work done in above equation

Where H is heat produced this is known as Joules law of heating effect.

NOTE:
1)according to joules law heating H=I^2*R*t

It is clear that formula heat is produced in a wire is directly proportional to the following :

·      Square of I

·      The resistance of wire R

·      Time ‘t’ for which fast

·      If we doubled the heat so thatr the electric current will be 4 times

·      In the same case they give us power ‘P’ and ‘t’ is time in that case heat energy can be calculated

P=W/t

P=H/t

H=P*t

 

§ APPLICATION OF HEATING EFECT OF ELECTRIC CURRENT

 

1)The heating effect of electric current is utilized in working of heating electrical appliances such as electric iron,room heater,etc.all the aplliances contain coil of high resistance made of nichrome of alloy (composition of nichrome is -nickel,Chromium,Magnese,Iron).

 

 

Large amount of heast produce in the heating coil made up of of nichrome having high resistrance but cord or connecting cable of electrical heater is made up of copper metal does not glow because copper metal have extremely low resistance.

2)A heating effect of electric current is utilized in electric bulb for producing light ,The filament of electric bulb is made of tungsten metal having high resistance and having melting point (3380⁰C).The filament should be thermally isolated as much as pass wool if air is present in the bulb filament will oxidized and burn quickly.

 

3)Another common application of Joules law heatimg ‘FUSE’.

Fuse protect circuit and appliances by stopping flow of any high electric current.

 

 

Q.The fuse is Placed in series with device?

Ans.Composition of fuse is a piece of wire short length of thin tin plated copper having low melting point.

 

 

Ø    ELECTRIC POWER

We know that rate of doing work is called Power.Electrical energy is used up for doing work .Electrical power is the electrical work done per unit time .

P=W/t

SI unit of Power is watt donated by ‘W’.

1 Power one watt is the rate of doing work of one joule per second .

1 Kilowatt=1000watt

Electric Power:The rate at which electrical work is done.

 

Formula for calculating Electric Power

 

We know that

 

P=W/t  -----------------------1

V=W/Q

W=V*Q    ---------------------2

I=Q/t

Q=It

By equation 2

 

W=V*I*t

By equation 1

P=VIP

 

 

P=I*R*I

 

P=I^2*R

P=V^2/R

Q.What is the meaning of 1KWH?

Ans.1Kwh is the amount of the electric energy consumed when the electrical appliances having a power rating of 1Kw is used for 1 hour.

1Kwh=3.6*10^6J